1. Field of the Invention
This invention relates to a process for preparing a ultrafiltration membrane suitable for the use in an artificial kidney belonging to the filtration type which has the selective permeability.
2. Description of Prior Arts
Most of the artificial kidneys conventionally employed belong to the so-called dialysis type artificial kidney that involves making the blood flow through a passage enclosed with a dialytic membrane and at the same time making a dialytic solution consisting mainly of a physiological saline solution flow along the reverse side surface of the dialytic membrane countercurrently to the blood, thereby removing low molecular waste products from the blood. The present method for removing the dissolved products by dialysis is suitable for removing urea or the like having a low molecular weight, but it is not satisfactorily effective for removing medium molecular weight products having molecular weights in a range of from 500 to 2,000, in which the toxic materials, recently being given attention as materials causing uremia, are included.
Removal of the medium molecular weight products can be effected by employing a membrane with an increased dialytic area or increasing the flow rate of the dialytic solution or the blood. However, these measures cause changes of composition or osmotic pressure of the intracellular fluid, intercellular fluid, cerebrospinal fluid, or the like, resulting in the occurrence of symptoms such as headache, vomiting, languorous feeling in the whole body, because the human body does not consist of a single compartment.
In contrast to the dialysis type, the artificial kidney belonging to the filtration type does not employ the dialytic solution, and it employs a system that involves; ultrafiltration of the blood to remove waste products having medium or lower molecular weights; addition of a substituting liquid (supplemental liquid) prepared by adding the constituents necessary for the human body to the blood containing concentrated amounts of blood corpuscles and proteinous materials produced by the ultrafiltration so as to dilute the concentrated blood until the normal concentration is reached; and returning the so adjusted blood to the body of the patient.
The ultrafiltration membrane to be used for the artificial kidney of the filtration type is required to prevent, as much as possible, the proteins in the blood from passing through the membrane and to give high permeant rate. Until now, there is not known a ultrafiltration membrane having such excellent characteristics. Since cellulose acetate has a stiff molecular skeleton with branching chains consisting of bendable substituents, it can be dissolved in a variety of solvents and is capable of precipitating as a filmy gel from a solution in a volatile solvent. It is knon that a thin membrane prepared by the above method is excellent as a dialysis membrane. This method can be also applied to the preparation of a thin membrane satisfactorily serving as the ultrafiltration membrane, if appropriate conditions for the preparation of membrane are selected.
The process for preparing a ultrafiltration membrane that employs cellulose acetate as the membrane material can be carried out by adding a swelling agent or a plasticizer to a solution of cellulose acetate in a volatile solvent and spreading the resulting solution. The nature of the selected swelling agent or plasticizer affect the conditions of gelation of the cellulose acetate. Accordingly, the pore diameter of the membrane greatly depends upon the so selected swelling agent or plasticizer. It is known that glycerol triacetate (as disclosed in Japanese Patent Publication No. 45(1970)-4,633) and a lower alcohol and its ester and a cyclic hydrocarbon and its ester (as disclosed in Japanese Patent Publication No. 48(1973)-40,050) can be employed as the swelling agent or the plasticizer used for this purpose. However, these additives are apt to remain in the membrane still after completion of the membrane preparation. Therefore, these are sometimes not appropriate for the preparation of the membrane for medical uses; and otherwise the formed membrane containing these additives does not satisfactorily show the characteristics of the ultrafiltration membrane.